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Dilley, a high school freshman, was found dead Sunday night in her second-floor bedroom at her mother’s home in Buna, Texas. Police suspect the teen was killed by 19-year-old college student Paul Audrey Adams, whom she had secretly been dating. Adams committed suicide the day after Tristan was shot twice in the head with a .22-caliber magnum revolver.

“Tristan is the type of person when she’s gone, you feel the void,” coach Danielle Wehmeyer tells PEOPLE. “What has happened to Tristan is unimaginable and has shaken our team to its very core. My girls will miss their teammate, their sister, and their friend. I will miss my cheerleader.”

Wehmeyer described Tristan as “an amazing athlete, student and friend” who loved cheerleading.

“Her teammates meant the world to her,” she said. “Cheerleading is a sport that requires trust and personality with Tristan we always had 100 % of both.”

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Tristan’s 13-year-old brother found her body, clothed, in a large pool of blood on top of the bed

Investigators discovered that at the time of her death she had been dating a boy named “Adam,” Jasper County, Texas, Sheriff’s Lt. Ryan Cunningham told PEOPLE — but neither her family nor her fellow cheerleaders knew much about him, other than his name and what they believed was his age – 16 not 19.

After searching through Tristan’s phone and Facebook, investigators learned that Adam’s true identity was Paul Audrey Adams, a 19-year-old nursing student at Lamar State College, less than an hour away.

Cunningham said he later learned that Adams had no criminal record and had been home-schooled.

“He didn’t have any friends,” he said. “He was home schooled. He had a friend growing up and that friend said he hadn’t talked to him in a couple of months. No one had any idea about his behavior.”

Some of the messages between Tristan and Adams showed they were planning to meet at her mom’s house on Sunday, the day she died, Cunningham says.

Tristan, who stayed with friends the night before, was home alone while her mom ran errands and her body was found shortly after 7 p.m. According to Cunningham, authorities believe she was killed sometime after 3:30 p.m.

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After obtaining evidence of a relationship between Tristan and Adams, investigators started looking for him themselves.

“I called dispatch and had them start pinging his cellphone,” Cunningham said. “We put out a BOLO out for him in the surrounding area. We were able to get a general location of the cellphone and we learned of his address,” in nearby Vidor, Texas.

Authorities arrived there about 1:30 a.m. on Monday, but he wasn’t home. His mother said he’d left the afternoon before, about 3:30, to meet “some girl in Beaumont he met at school,” Cunningham said.

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“Both kids were deceiving the parents,” he said. “He [Adams] knew he was too old to be seeing the child.”

At 10:30 a.m. Monday, detectives got a call from a Texas Ranger who told them he had gone back to interview Adams’ mother and was talking to her when Adams called her, Cunningham says.

“The Ranger listened for a few minutes and took the phone,” he said. “He tried to get Paul to come in and speak to us, but he didn’t want to come in.”

Cunningham alleged that, in that phone conversation, Adams admitted to the Ranger that he had been over at Tristan’s house the day she died when they heard an intruder coming upstairs.

“He gave a description of an older white male with a beard,” Cunningham said. “He said he hid in the shower and he could hear Tristan screaming, ‘Get off of me.’

“He said he heard two gunshots and heard the man running out of the house. He waited a couple of minutes and then found Tristan dead and he didn’t know what else to do, so he ran.”

RELATED VIDEO: Cheerleader’s Brother Finds Her Dead in Bed — Leading to Discovery of Her Secret Relationship with an Older Teen

After Adams told the Ranger this story, the Ranger then “heard a distinct gunshot and he believed Paul had shot himself,” Cunningham said.

He said they started searching for Adams when they came upon a tent and his white ’94 Toyota around noon on Monday, next to a canal just east of Vidor.

They were about 150 yards away from the vehicle and continuing to approach when they saw Adams — and saw him reach for a gun.

“As soon as me and my partner said, ‘I think he has a gun,’ we observed the suspect shoot himself in the head and collapse,” Cunningham said.

Cunningham said detectives learned that a few days before Tristan’s killing, Adams bought an axe, a machete, a hatchet, several knives, 32 meal rations, cases of water, burlap sacks, dumbbells and duct tape. He had also purchased several sets of handcuffs and chains.

“It was a premeditated event,” Cunningham said.

Detectives also discovered that Adams had downloaded apps from his phone and had been listening to police dispatch traffic as they investigated Tristan’s death, according to Cunningham.

“I think he knew we were coming,” he said, “but I don’t think he realized it was going to be quite so fast.”

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With his story of a violent intruder, Cunningham said, Adams was just trying to cast blame elsewhere: “Why would you not immediately notify law enforcement? Why would you run and go to the extent of buying all of these items? It was pretty clear-cut. You could read between the lines.”

Still, even with what authorities now know, it remains unclear how and when Tristan and Adams first met — or what motivated him to kill her.

“It was senseless, and it makes no sense,” Cunningham said. “There are so many questions we can’t answer. We will never know what was going on in his mind and what was said between them.”

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